Vapor compression refrigerators achieve maximum efficiency when the evaporator, in which liquid refrigerant is vaporized by heat absorbed from the refrigerated space, is supplied at its inlet with an optimum mass flow of liquid refrigerant that is just sufficient so that vaporization is complete at the evaporator outlet. Flow in excess of the optimum results in liquid refrigerant leaving the evaporator outlet, thereby sacrificing its refrigeration capability.
Flow less than optimum results in complete vaporization occurring within the evaporator. Between the point of complete vaporization and the evaporator outlet, vapor is "superheat", as used in reference to vapor compression refrigeration, means the difference between the temperature of vapor at some point in the suction live downstream of the evaporator and the temperature of the liquid-vapor mixture at the evaporator inlet. High superheat is a source of inefficiency because only part of the evaporator is available to absorb heat by efficient heat transfer from the refrigerated medium to boiling liquid refrigerant. The remaining part transfers heat inefficiently from the refrigerated medium to refrigerant vapor. The result is that superheat causes the evaporator to operate at lower than optimum temperature and pressure, requiring more compressor work per unit of refrigeration.
Nearly optimum flow of refrigerant has been achieved in prior art with electronically controlled expansion valves (EEVs). Some prior art EEVs regulate refrigerant flow with an electromechanically adjustable flow resistor such as a needle valve. In others, an electromechanical valve periodically opens to admit flow to a fixed orifice for a controllable time interval.
In prior art, an EEV is part of a closed loop feedback control in which superheat is sensed by temperature sensors, and a superheat signal controls an EEV so as to increase refrigerant flow when superheat temperature increases above a preset value and reduce refrigerant flow when superheat falls below the preset value. Since increased flow reduces superheat, the system has negative feedback and will, if it is stable, settle at or near the preset superheat. The value of preset superheat is typically below 7 degrees Centigrade, which is low enough so that most of the evaporator is used efficiently.
In an EEV control loop, a step increase in flow rate at the evaporator input generates a corresponding step increase in flow rate at the evaporator output after a delay equal to the time required for refrigerant to transit the evaporator. This delay is typically about 10 seconds, and has serious implications for control loop stability, as may be seen from the following sequence of events in a "proportional only" EEV control in which change in flow rate is simply proportional to change in superheat.
Suppose that a "proportional only" system has been running with preset superheat, and at time=0, a disturbance such as a momentary interruption of power, causes superheat to increase well above its preset value. Then, at time=0, the EEV will automatically be stepped to high flow rate in an attempt to restore preset superheat. Assuming a delay time of 10 seconds, the step increase in flow results in liquid refrigerant reaching the output temperature sensor at time=10 seconds. In a short time interval prior to and after the arrival of liquid at the output temperature sensor, the sensor temperature and consequently the superheat signal both decrease, and the controller reacts with an abrupt decrease in flow rate at the evaporator input. However, this decrease does not reach the output temperature sensor until time=20 seconds, at which time the superheat signal abruptly increases and the foregoing sequence begins to repeat itself.
In prior art, EEV controls have been stabilized electronically by empirical adjustment of a "PID" (proportional-integral-differential) controller (Ref. 1, FIG. 2)., which typically results in slow controller response and low margins of stability. Also, the cost of a PID controller precludes its use in many applications.
Accordingly, the purpose of the present invention is to provide inexpensive stabilization an EEV control loop so as to achieve a high margin of stability and relatively fast controller response with "proportional only" control.